


Lies My Sister Told Me

by Anonymous



Category: Gravity Falls
Genre: Aged-Up Character(s), F/M, Incest, Jealousy, Mild Smut, Mildly Dubious Consent, Oral Sex, Sibling Incest
Language: English
Status: Completed
Published: 2016-02-27
Updated: 2016-02-27
Packaged: 2018-05-23 13:40:57
Rating: Mature
Warnings: Underage
Chapters: 1
Words: 1,739
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6118171
Author URL: https://archiveofourown.org/
Summary: <blockquote class="userstuff">
              <p>"I'm not jealous," Mabel said, biting her lip and looking at the floor, once again avoiding meeting eyes with her brother.</p><p>Dipper had been examining his sister for quite some time now. Everything about her body language screamed otherwise.</p>
            </blockquote>





	Lies My Sister Told Me

It was Dipper and Mabel's fourth summer at Gravity Falls.

Over this period of time, Dipper and Mabel, as well as the people around them matured significantly. Friendships grew, fights were had, monsters were destroyed...despite how hesitant they were about the place on their first visit, and those couple of occasions they almost died, Dipper and Mabel wouldn't have it any other way.

However, no bond grew more over the four summers than the nexus between Dipper and Candy, one of Mabel's friends she had met during her first summer in Gravity Falls. Candy had briefly crushed on him during their first summer there, but then quickly was over it due to Dipper's massive screw-up of flirting with multiple girls.

But that crush quickly came back to Candy upon their second summer in Gravity Falls, Dipper having gotten taller, a tad bit hairier, and a hell of a lot finer. Maybe those were just her hormones waking up.

Candy had pushed herself to spend more time around Dipper that summer, using her natural charm to show that she was interested again. And it certainly worked for Dipper, though the two didn't officially start dating until their third summer at Gravity Falls. 

Since then, Dipper and Candy had been in a strong but long-distance relationship, fueled through the power of technology. 

There was one thing Dipper couldn't help but notice any time he was talking to Candy, flirting with Candy, or even mentioning Candy with Mabel in the room. His sister suddenly tuned out, like she couldn't handle him talking about her or something.

But that was crazy. She and Candy were still friends, so...what was the deal?

This behavior seemed to only intense upon their fourth summer in Gravity Falls, and at this point Dipper was fueling it on purpose, trying to figure what exactly was going on with his sister.

* * *

On this particular night, Dipper was getting dolled up for a date with Candy, wearing the finest clothing his little income could purchase. It was the first date that they had been on since their last summer in Gravity Falls, and they were both obviously excited to be able to see, talk with, and touch each other physically again.

"Mabel, do you think this is too dressy?" Dipper asked his sister, who was lying on their bed with a coloring book. "Ditch the tie?"

"Yeah, uh, sure, whatever." Mabel muttered half-heartedly.

"You didn't even look!"

"Do what you want, man."

Dipper sighed. "Mabel, _what_ is the deal?"

"What?" Mabel questioned, looking up at her brother. "What deal? Who has a deal? I don't have a deal."

"You obviously do." Dipper replied. "Anytime I so much as mention going out with Candy, you get really awkward and quiet and avoid eye contact with me."

"Not true!" Mabel exclaimed. 

"You're not even looking at me right now!"

It was true. Mabel was looking down at her coloring book again and was finding it hard to look up at her brother every ten seconds or so.

"Mabel, are you uncomfortable with me going out with Candy?" Dipper asked. "Is it because she's your friend? I get how that can be kinda weird, but-"

"No!" Mabel exclaimed. "You're crazy, Dipper Pines! That is not it! I've dated a few of your friends too, remember?"

"Ugh, Josh. Don't remind me." Dipper muttered. "I admit I felt awkward then, him coming over to play video games and the night ending with you two making out and ignoring me. But if that's not the problem, then what is?"

"Just leave me alone." Mabel sighed, going back to her coloring book. "I don't want to talk about this, okay? Have fun on your date."

Mabel forced another second of eye contact with Dipper, and this time, he noticed that her eyes were on his lips.

Everything clicked in his head at once, and he got a baffled expression on his face.

"Mabel...don't tell me," Dipper began, "you're jealous?"

Mabel jumped up from the bed all at once, leaping over to her brother and slapping him in the face. "What? Bullshit! Jealous? Are you out of your mind? I'm your sister!"

Dipper jumped in alarm. He had  _never_ heard his sister swear before.

"Alright, alright," Dipper replied, holding his hands out and trying to calm his sister down. "I'm sorry. I guess that was kind of silly of me to accuse you of. We're twins, after all. That'd be really weird."

However, Dipper noticed how Mabel's body tensed up after Dipper stated 'that'd be really weird'.

Dipper suddenly began to have a flashback. When he was thirteen, Mabel had walked in on him at his computer, pants around his ankles, logged into a generic porn site, with the title clearly displayed in a bold Arial font:  **"Brother and sister first time".**

Mabel bolted from the room as quickly as she opened the door, and Dipper had felt really awkward around her for the next couple of days. He didn't normally look at porn like that. It was just in the related videos and he was...well, curious. A brief lapse in judgment if you will.

But if anything, Mabel had acted even friendlier towards him in those couple of days. One could even call it...playful.

How could Dipper had missed this obvious sign before?

Mabel had been hiding her emotions for so long, and they were finally spilling out.

"You tensed up there," Dipper stated, sitting on the bed next to his sister. "Mabel, I won't be mad if you're honest with me. Are you jealous?"

Glancing up again at her brother, she gave him a stern look.

"I'm not jealous," Mabel said, biting her lip and looking at the floor, once again avoiding meeting eyes with her brother.

Dipper had been examining his sister for quite some time now. Everything about her body language screamed otherwise.

The way she kept crossing and uncrossing her legs from her skirt, that look she had given at his lips...

"Okay, then," Dipper began, an evil smirk emerging on his face, "then you won't mind if I detail the events of tonight, will you?"

Mabel gulped.

"Well, first we're going to see  _Slaintub,_ that new superhero movie where he's actually super raunchy. Candy said she'd been wanting to see it for quite some time." Dipper said. "But after that, we're going to head back to her place."

Dipper crept up towards his sister, mischievously whispering in her ear. "We've spent so much time away from each other, just dying for each other's touch..."

"Stop it." Mabel muttered, pushing her brother away, her face bright red. "Just what do you think you're doing?"

"Are you going to admit you're jealous now, or do I have to keep going?" Dipper asked.

" _I'm. Not. Jealous."_

"Suit yourself." 

Dipper came back to his sister's ear, breathing hard on her as his tone became seductive. "I'm going to begin kissing her as she strips, making sure I hit every inch of her body. Head to toe."

Mabel tensed up, feeling a mixture of tingly and awful all over. Dipper poked her cheek and came as close as humanly possible.

"She's going to rip my clothes off then, the feeling of ecstasy taking over her whole body." Dipper continued. "She's going to let me-"

" _Dipper Pines,"_ Mabel interrupted in a moan,  _"I need you to stop."_

"Or else what?" Dipper teased, giving his sister a kiss on the cheek. "What are you going to do?"

Just like that, Mabel grabbed her brother and began passionately kissing him, her impatient lips meeting his for the first time in such a context. She pushed Dipper onto her as her inner desire took over versus her usual morality.

"That's what I'm going to do," Mabel replied. "That and much more."

"Well, maybe that's what I want." Dipper stated, traveling below his sister's upper body down to her skirt.

"What are you doing?" Mabel questioned.

"Just following orders."

Without any hesitation, Dipper ripped her knickers off using only his teeth, something Mabel found both erotic and impressive. Her eyes started to roll into the back of her head as a feeling of bliss took over.

"Dipper..." Mabel moaned. "Is the door locked?"

Dipper stopped for a brief second. "Made sure it was locked when I came in here."

"So you were planning this?"

"Just a gut feeling."

"How..." Mabel moaned, unable to think properly or construct a full sentence in her head.

"Shh." Dipper purred, not wanting to hear anything but her moans of pleasure as he engulfed her.

"Oh, fuck," Mabel cursed. "Dipper, how are you...why are you..."

Dipper had never realized how sexy it was to hear Mabel use profanity before now, simply because it was almost as taboo as what they were doing.

"I'm...falling..." Mabel moaned, jerking and feeling herself reaching the end of her euphoria.

Mabel reached her peak, moaning intensely as Dipper did exactly what Mabel never expected him to do. 

Candy was so lucky.

"That was quick." Dipper teased, sliding Mabel's underwear over her legs and pulling her skirt back up.

"Don't be rude," Mabel replied with a smirk, still breathing heavily. "My mind's still processing what happened."

Mabel paused, then playfully slapped her brother. "You son of a bitch, I feel guilty now. We don't have time for me to return the favor, do we?"

"When did you become such a sailor mouth?" Dipper asked. "I like it."

Dipper chuckled. "But think about what you just called me."

"Huh?" Mabel asked. "What do you me... _ohh_."

 _'Son of a bitch'._ They had the same mother. Mabel had honestly forgotten for a second.

"Get it now?" Dipper asked, smiling at his sister, standing up and adjusting his tie. "You should probably hit the shower."

"Was planning on it," Mabel said. "This isn't my first time, you know. I know the basics."

"Hmm, it's not?" Dipper asked with a frown. "I'm going to need more details on these other times when I get back tonight. A lot of details."

"Can do," Mabel replied, going back to her coloring book like nothing happened. However, Dipper noticed that things were significantly less awkward between them now.

 _"Dipper, your girlfriend's here!"_ Grunkle Stan suddenly yelled from the other room.

"Have fun tonight," Mabel said with a smile.

"I will." Dipper replied, heading out the door of their room. "And so will you."

**Author's Note:**

> Even though I'm posting this anonymously from an account no one IRL knows about I'm still paranoid someone IRL will find this and try to get me thrown in therapy...disclaimer: I do not want to engage in intercourse with any of my actual family members. Awkward conversation avoided.
> 
> Still new to smut, hope that this was decent enough. :)


End file.
